Refrigeration control system



June 10, 1941; NEWTON 2,244,892

REFRIGERATION CONTROL SYSTEM Filed Jan. 20, 1959 l6! inventor Ottorngg Patented June 10, 1941 2,244,892 REFRIGERATION CONTROL SYSTEM Alwin B. Newton, Minneapolis, Minn., assignor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn., a corporation of Delaware A plication January 20, 1939, Serial No. 251,993

17 Claims.

This invention relates to refrigerating apparatus and more particularly to a control system for ya refrigerating apparatus that automatically causes defrosting of the evaporator when conditions require it.

It has heretofore been proposed to initiate a defrosting cycle of the evaporator between each operation of the compressor by preventing restarting of the compressor until the suction pressure in the system rises to such a value that the evaporator is completely defrosted. Such a control system has been found to be very satisfactory in most cases. However, when the fixture being cooled is 'to be maintained at a very low temperature, such as F., it may not be possible to initiate a defrosting cycle between each operation 01 the compressor and maintain this low temperature in the fixture, and it is to this type of a refrigerating apparatus that my invention is particularly directed.

In accordance with the teachings of my invention, a controller responsive to the temperature in the fixture normally controls operation of the compressor to maintain this temperature at desired values. Provision is made for interrupting operation of the compressor should the head pressure in the system become excessively high and to prevent restarting of the compressor until this head pressure drops to a predetermined value to reduce the starting load on the compressor motor. Provision is also made for interrupting operation of the compressor should the suction pressure drop to an excessively low value, and when the compressor is stopped by reason of the suction pressure becoming excessively low it cannot be restarted until the suction pressure rises to a sufficiently high value to cause defrosting of the evaporator. However, when the compressor is stopped by reason of the temperature in the fixture dropping to the required value, it is not necessary for the suction pressure to rise to the defrosting value so that defrosting of the evaporator'occurs only when the compressor is stopped by reason of the suction pressure becoming excessively low without causing the fixture temperature to drop to the desired value. 1

Provision is made for isolating that portion of the fixture containing the evaporator from the storage compartment of'the fixture during the defrosting operation so that the-temperaof is believed to be unnecessary.

ture of the storage compartment will not rise ex- I cessively during the defrosting period. At the same time, that portion of the fixture containing the evaporator is opened to the atmosphere and provision is made for blowing heated air over the evaporator at this time to accelerate the defrosting thereof. As soon as the evaporator has been defrosted, the compressor will be restarted and communication reestablished between the two portions of the fixture so that the temperature in the storage compartment may be reduced to the desired value.

It is therefore an object of my invention to provide a refrigerating apparatus of the type above described wherein provision is made for automatically defrosting the evaporator when conditions require that this be done and at the same time for isolating the storage compartment of the fixture from the evaporator to prevent undue rise in temperature therein and at the same time to accelerate the defrosting of the evaporator by blowing heated air thereover, there also being provision for interrupting operation of the compressor when the head pressure becomes excessively high'and for preventing restarting of the compressor at all times until the head pressure has dropped to a predetermined value.

Other objects and advantages will become apparent upon reference to the specification, claims and appended drawing wherein is illustrated a preferred form of my invention.

Referring more particularly to the drawing, a fixture I0 is provided with an evaporator H in the upper portion thereof, the lower portion of I the fixture l0 having a storage compartment l2 within which meats or any other articles that are to be maintained at a relatively low temperature, below 32 F., such as 10 F., may be stored. The evaporator ll forms a part of a mechanical refrigeration system of a conventional type which system also includes a compressor l3 driven by a motor M; the compressor being connected by a-pipe I5 to a condenser l6 leading to the receiver l1 whence condensed refrigerant flows through a pipe I8 to the evaporator I I under'the control of a conventional thermostatic type expansion valve IS. The evaporated refrigerant then fiows back to the compressor by way of the pipe 20. The operation of such a refrigeration system is well understood in the art and further description there- Suitably located within the fixture I0 is a heating element 22 anda fan or blower 23 driven by a motor 24, these elements being so arranged that when the heating element 22 and the motor 24 I are energized the fan 23 will cause a circulation of heated air over the evaporator H. Located below the evaporator is a drip pan 25 having an outlet 25, this drip pan being arranged to catch the moisture which is melted from the evaporator I I during the defrosting period, the moisture then passing from the drip pan by way of the drain 26. The upper portion of the fixture Ill is provided with openings 21, 28 which are normally closed by the doors 29 and 30, respectively, the door 29 being connected by links 32 and 33 to the arm 34 of a motor 35 and the door being connected by the links 31, 38, 32, and 33 to the arm 34 of the motor 35. These linkages are so arranged that when the arm-34 of the motor completes a half revolution, the doors will move from the full line positions to the dotted line positions wherein they close the spaces formed between the drip pan 25 and the side walls of the fixture Ill thus isolating the upper portion of the fixture containing the evaporator II from the lower portion or storage compartment I 2. At the same time, the openings 2'! and 28 permit communication between the upper portion of the fixture and the atmosphere to accelerate the defrosting of the evaporator.

The reference character illustrates a control device for controlling the operation of the compressor motor, this control device being of the type illustrated in application Serial No. 196,447 of Albert L. Judson and Carl G. Kronmiller, filed on March 17, 1938. This control device generally comprises a base II on which is mounted a bellows 42 connected by means of a pipe 43 with the suction line 20 of the refrigeration system. This bellows acts on a lever 44 pivoted at 45 andhaving its opposite end connected by a spring 46 to a nut 47 mounted on an adjusting screw 48, this adjusting screw being provided for varying the tension of the spring 46 and the calibration of the bellows 42. The lever 44 carries a bridge member 50 by means of the insulating pad 5|, this bridge member including contacts 53 and 54. The contact 53 cooperates with a contact arm carried by the terminal 55 and the contact 54 cooperates with a contact arm 51 carried by the terminal 58. The positions of these contact arms 55 and 51 when disengaged by the contacts 531 and 54 may be varied by means of the concentrically arranged cams 58 and 59. With the parts in the positions illustrated, it may be assumed that the suction pressure is at 35 pounds andthe evaporator has just been defrosted and at this pressure the contacts 53 and 54 are in engagement with the contact arms 55 and 51. As the suction pressure drops below. 35 pounds, the contact 53 moves out of engagement with the contact arm 55 and upon a drop in the suction pressure below 5 pounds, for example, the arm 51 will be moved into engagement with the cam 59 and the contact 54 will then move out of engagement with the contact arm 51.

Also carried by the base 4| is a bellows 62 connected by means of the pipe 63 with the high pressure line I5, this bellows acting upon a lever 65 pivoted at 65 and having its other end connected by means of the spring 51 to the nut 58 carried by the adjusting screw 69. .Adjustment. of this screw 69 adjusts the tension of the spring Gland in this manner the calibration of the bellows 62 may be adjusted. Carried by the lever 65 is an adjustable abutment member I2 having abutments I3 and I4. The abutment I3 is adapted to engage a contact member I6 carried by a terminal I1 and the abutment member I4 is adapted to engage a contact member 19 carried by the terminal 55. The contact member 16 engages a contact member 80 and the contact member 19 engages a contact member 8|. Upon an increase in pressure in the line I5, the bellows 52 expands and the abutment member I4 moves into engagement with the contact arm I9 and causes movement of this arm out of engagement with the contact 8| at a pressure of 135 pounds, for example. Upon a further increase in pressure in the line I5, the abutment member I3 will engage the contact arm I5 at 185 pounds, for example, and cause movement of this contact member away from the contact 80. Upon a decrease in pressure below 135 pounds, the arm I5 will move back into engagement with the contact and when the pressure drops below 135 pounds, the contact arm 19 will move into engagement with the contact 8|.

The reference character 35 indicates a relay or starter for the compressor motor I4. This relay or starter comprises a relay coil for operating a bridge member 81 with respect to the maintaining contacts 88 and 89 and a second bridge member 90 with respect to the load contacts 9| and 92. Upon energization of the relay coil 86, the bridge members will move into engagement with their respective contacts and upon deenergizatlon of the coil 36, the bridge members move out of engagement with their respective contacts under the influence of gravity or any suitable biasing means (notshown).

The reference character indicates an overload cut-out mechanism which includes a heater element 95 connected between a terminal 31 and the load contact 9| for heating a thermostatic element which trips open contact 99 and IIIII upon the occurrence of an overload condition. The contacts 99 and I00 may be manually reclosed by means of the reset arm IlII The operation of this control device 40 for the compressor motor I4 will be more fully explained as the description proceeds.

The reference character H5 indicates generally a thermally operated switching mechanism. This relay comprises a bimetallic element III carrying bridge members H2 and H3 suitably insulated from the bimetallic element III, this bimetallic element being connected by means of link II4 to a second bimetallic element H5 carrying bridge elements IIS and II! suitably insulated from this bimetallic element. when the bimetallic elements are in the positions shown. the bridge element I I2 closes a circuit through the contacts 9 and I20 while the bridge element II3 closes a circuit through the contacts In the other position which these I2I and I22.- elements are adapted to assume, the bridge elements H2 and H3 move out of engagement with their respective contacts and the bridge element II6 carried by the bimetallic element II5 closes a circuit through the contacts I24 and I25 while the bridge member III closes a circuit through contacts I25 and I21. Located adjacent the bimetallic element III is an electrical heating element I30 and a second heating element I3I is located adiacentthe bimetallic element I I 5,there being an insulating wall I32 separating these heating elements so that one heating element will'not affect the bimetallic element on the opposite side of the wall I32. When the heating element I30 is energized, the heat imparted thereby to the element III causes this element to move to the right to move the bridge members out of engagement with their respective contacts and by reason of the link II 4 the element H5 is moved in the same direction. Conversely, when the heating element I3! is energized, the bimetallic element moves towards the left causing similar movement of the bimetallic element III. In order to cause the elements to move with a snap action, magnets I33 and I34 are located adjacent the bimetallic elements III and 5, respectively. The magnet I33 prevents movement of the element III until suilicient stress has been set up in this ture III, the internal parts of this motor have been illustrated generally by the reference character I40. This motor includes an armature HI and a field winding I42, the armature I being connected by suitable reduction gearing I43 to the shaft I44 which carries the arm 34 for operating the doors 29 and 30. The shaft I44 also carries a contact arm I45 which cooperates with the contacts I46, I41 and the contact ring I48 in a-manner to be hereinafter described. The

shaft I44 may also carry a cam member I50 which engages a lever |5| pivoted at I52 and carrying the mercury switch I53. As the shaft I44 rotates through 180 to move the doors 28 and 29 from their full line positions to their dotted line positions, the cam I50 will likewise rotate through 180 whereupon the switch I53 will be tilted to a. position wherein the terminals I55 and I56 are bridged by the mercury element of the switch I53, as will be understood from the drawing.

Located within the fixture- I2 is the thermostat I50, this thermostat including a bellows |6| having a suitable volatile fill whereupon the bellows'will expand or contract in accordance with The bellows |6| acts upon a lever I62 pivoted at 5 I63 and biased into engagement with the upper portion of the bellows by means of the spring I64. A mercury switch I65 is carried by the lever I62. This switch includes three terminals I61, I68, and I69 which are adapted to be connected together by the mercury element of the switch when in the position illustrated. When the temperature within the fixture drops to a sutllciently low value, the switch I65 will be tilted in the opposite direction thus breaking the contact between the terminals of the switch. Operation With the parts in the position illustrated, it

may be assumed that a defrosting operation has just been completed, that the temperature within the compartment I2 of the fixture I0 is above the desired value which may be taken as 10 F., that the suction pressure within the refrigeration system is above 35 pounds so that the contacts 53 and 54 of the bridge member 50 are in engagement with the contact arms 55 and 51 and that the head pressure is below 135 pounds so that the contact arms 16 and 1.9 are in engagement with the contacts 80 and 8|. With the parts in these positions, the relay coil 86 of the variations in temperature within the fixture I2.-

the terminal 80, contact member 16, terminal 11, wire |8|, terminal 58, contact members 51 and 54 through the bridge member 50, contact members 53 and 55, 19 and 8|, conductor I83 through the terminal 89, conductor I84, the overload cut-out contacts 99 and I00, relay coil 86, conductor I86 to the power terminal l81 and thence to the line I88, these line wires I80 and I88 being connected to a suitable source of power, not shown. The energization of the relay coil 86 causes the bridge members 81 and 90 to move into engagement with the maintaining contacts 88 and 89 and the load contacts 9| and 92, respectively.

Movement of the bridge member 90 into engagement with the contacts 9| and 92 closes a circuit to the compressor motor I4 as follows: from the line I80, the terminal 80, conductor 200, terminal 91, overload resistance member 96, contact 9|, bridge member 90, contact 92, conductor 20I, through the motor I4, conductor 202 to the power terminal I81 and line I88.

Movement of the bridge member 81 into engagement with the contacts 88 and 89 establishes the following holding circuit for the relay coil 86: from the line wire I80, contact members 80 and 16, terminal=11, conductor I8I, terminal 58, contact members 51 and 54, bridge member 50, conductor I90, contact I2I of the switching mechanism IIO, bridge member II3, conductor |9| to the terminal I69 of the switch I65, through the switch to terminal I61, conductor I92, the maintaining contact 88, bridge member 81, contact 89, conductor I84, overload contacts 99, I00, conductor I96, relay coil 86, conductor I86, power terminal I81, and line I88. This maintaining circuit, it should be noted, does not include the suction pressure contacts 53 and 55 or the high pressure contacts 19 and 8|. In other words, as long as the suction pressure contacts 54 and 51 remain closed and the high pressure contacts 16'and 80 remain closed indicating that the suction pressure is above 5 lbs. and the head pressure is below 185 lbs., the compressor will remain in operation as long as the temperature in the fixture is above the desired value, once the compressor has been put into operation. Should the temperature in the fixture now drop to the desired value thus opening the circuit through other.

control device 40 will be energized through the the contacts I61 and I69, the maintaining circuit through the relay 86 will be interrupted. In or-- der for the-temperature in the storagecompartment to drop to this desired value by operation of the evaporator II, it is necessary that the suc-,

tion pressure drop below 35 lbs. in order that the evaporator will have the desired cooling effect so that the original energizing circuit for the starting coil 86 will of necessity have been interrupted and the compressor will now stop, thus placing the refrigerating system out of operaion.

The temperature in the storage compartment I2 will gradually rise above the desired value thus closing the contacts I61 and I69 of the switch I65 and the compressor I3 will again be placed in operation, but only after'the head pressure has dropped below 135 lbs. so that contacts 19 and 8| are closed. It is not necessary at this time, however, for-the suction pressure to rise to 35 lbs. so that it is possible to start the compressor even though the contact members 53 and 55 are not in engagement with one an- The circuit for energizing the relay coil 86 at this time is as follows: from the line wire I through the contacts 80 and 16, terminal 11,

conductor I8I, contact members 51 and 54, bridge member 50, conductor I90, contact I2I of switching mechanism I10, bridge member H3 conductor IQI, terminals I69 and I68 of the mercury switch I65, conductors I95, I96, terminal 56, contact members 19, 8I, conductor I03, contact 89, conductor I84, contacts 99 and I00, conductor I96, relay coil 86, conductor I86, power terminal I61 to the line wire I88. It will thus be seen that the'relay coil I36 will be energized upon a call for cooling by the thermostat I60, even if the suction pressure is below 35 lbs. so that the contact members 53 and 55 are not in engagement with one another, as long as the contact I2I of the switching mechanism 110 is in engagement with the bridge member I I3. Accordingly, after each operation of the compressor by reason of the thermostat I60 becoming satisfied, the compressor will be restarted upon a further call for cooling after the head pressure drops to 135 lbs, this provision being made to reduce the starting load on the compressor motor I41 After the compressor has been operated for a suflicient number of cycles, the frost on the evaporator II may become sufficiently thick so that the storage compartment I2 in the fixture I is not properly cooled. As the frost builds up on the evaporator II, the suction pressure will gradually drop and if the suction pressure drops to 5 lbs., for example, without causing the temperature in the fixture to become low enough to satisfy the thermostat I60, the contraction of the bellows 42 will cause movement of contact member 54 out of engagement with contact member 51. The original maintaining circuit which was described above is now interrupted at these contacts and sufficient current now flows through the following circuit to cause the heating element I30 of the switching mechanism Hi] to heat up, this circuit being as follows: from the line 'wire I80 through contact member 80, contact member 16, terminal 11, conductor I8I, terminal 58, conductor 205, heating element I30, conductor 206, contact I22, bridge member H3, conductor I9I, terminal I69 of the mercury switch I65, terminal I61, conductor I92, maintaining contact 88, bridge member 81, contact 99, conductor I84, contacts 99, I00, conductor I96, relay coil 86, conductor I86, power terminal I81, and line I80. This circuit was, of course, established upon energization of the relay coil 86 but sufficient current did not flow through this circuit while the original maintaining circuit for the starting coil which was described above was established since these circuits were in parallel and the heater was eifectively shunted out by contacts 54 and 51. However, upon the opening of contacts 54 and 51, the only maintaining circuit for the relay coil 86 is through the heater I30 so this heater now begins to heat up and at the same time heat up the bimetallic element III After a short time, sufi'icient heat will be imparted to the bimetallic element III to cause 4 this element to move to the right overcoming the attraction of the magnet I33 and at the same time the bimetallic element II5 moves to the right, this movement taking place with a snap action so that the bridge member II3 moves out of engagement with the contact I2I and the bridge members H6 and H1 carried by the bimetallic element II5 move into engagement with their respective contacts.

The maintaining circuits for the compressor motor I4 are now both interrupted as isthe original energizing circuit by reason of the drop in suction pressure so that the compressor now shuts down and the flow of refrigerant through the evaporator is thus interrupted. The circuit through the heater I30 is also interrupted at this time by the movement of bridge member H3 out of engagement with the contact I22. Movement of bridge member I I6 into engagement with contacts I24 and I25 causes-the field winding I42 of the motor 35 to be energized as follows: from the line wire 220 through conductors MI and 222, contact I24, bridge member II6, contact I25, conductor 224 to the contact I46, arm I45 carried by the motor shaft I44, conductor 226, field winding I42 and conductor 221 to the line wire 230. Energization of the field winding I42 causes rotation ofthe armature I and through the reduction gearing I43 the shaft I 44 is caused to rotate in a clockwise direction. As the shaft I44 rotates, the arm I45 moves out of engagement with the contact I46 thus interrupting this energizing circuit for the field. winding I42 but as the arm I45 moves out of engagement with contact I46, it moves into engagement with the contact ring I48 thus establishing a maintaining circuit for the field winding I42 as follows: from the line wire 220 through conductors 232, 223, contact ring I48, arm I45, conductor 226, field winding I42, and conductor 221 to the line wire 230. It will thus be apparent that as long as the arm I45 remains in engagement with the contact ring I 48, the field wind- .ing I42 will be energized. After the arm I45 has moved through however, it moves out of engagement with the ring I48 and into engage-- ment with the contact I41. As the arm-I45 moves out of engagement with the ring I43, it interrupts the circuit through the field winding I 42 and the armature I4I accordingly stops rotating which in turn causes the shaft I44 to stop rotating.

The shaft I44 has now rotated through the 180 degrees, thus causing rotation of the arm 34 carried by the shaft through the 180 degrees and moving the doors 29 and 36 from their full line positions into the dotted line positions. Thesedoors now cooperate with the drip pan 25 in separating the fixture I0 into two isolated compartments. At the same time, the upper compartment is placed in communication with the outside atmosphere by way of the openings 21 and 28. The cam I50 carried by the shaft I44 is also moved through the 180 degrees thus tilting the switch I53 to close the contacts I 55 and I56. A circuit is now established through the heating element 22 as follows: from the line wire 230, conductor 235, switch I53, conductors 236, 231, heating element 22, conductors 238, 239, and 232 to the line wire 220. :At the same time, the blower motor 24 is energized by way of the following circuit: from the line wire 230, conductor 235,'switch I53, conductors 236, 240, motor 24, conductors 24I, 239, and 232 to the line wire 220. The compressor has now been stopped, the doors 29 and 30 moved to their dottedline positions to isolate the storage compartment I2 from the evaporator II, the upper compartment in the fixture I2 is now in communication with the atmosphere, the heating element 22 is energized, and the fan 23 is rotat ing and. causing air to circulate over the heating element 22 and the evaporator II to cause rapid melting of the frost therefrom. Since the storage compartment I2 is separated or isolated from the evaporator II, this defrosting operafrom the evaporator II, the water dripping into the pan 25 and running out the drain 26. The' suction pressure will now rise rapidly to 35 lbs.

whereupon the contacts 53 and 54 carried by the bridge member 58 move "into engagement with the contact members 55 and 51. By this time the head pressure will also have fallen suiilcieintly so that the'contact member 18 is in engagement with the'contact 8I. The original energizing circuit through the starting coil 88 which did not include the thermostat I68 will now be established since all of the contacts controlled by the bellows 42 and 82 will be closed. The following circuit will also be established through the heating element I3I; from the line wire I88, through contacts 88, 18, terminal 11, conductor I8I, terminal 58, contact members 51, 54, 58, 55, terminal 58, conductors I96, 258, heating element Ill, conductor 25I, contact I21, bridge member II1, contact I26, conductor I28, power terminal I81, and line wire I 88. Suiiicient current will now flow through the heating element III since this heating element is connected directly across the line wires I88 and I88, so that the bimetallic element I I 5 will become sufficiently heated to cause movement of this element and the bimetallic element III to the left thus moving the bridge members H6 and H1 out of engagement with their respective contacts and breaking the circuit through the heating element III while at the same time moving the bridge elements H2 and III into engagement with their respective contacts where they are held by the magnet I38. Since the thermostat I 88 is now calling for' cooling, it being recalled that "as follows: from the line wire 228 through conductors 22I, 255, contact H9, bridge member II2, contact'I28, conductor 258, contact I41, arm I45, which it will be recalledcame to rest in en- I aagement with this contact, conductor 228, fleld winding I42 and conductor 221 to'the line wire 258. K The armature I will again start rotating, moving the arm I45 out of engagement with the contact I41 but into engagement again with the contact ring I48, thus reestablishing the maintaining circuit for the field winding I42 and causing rotation of the motor until-it returns to its original position as illustrated where the field winding will again be deenergized since the bridge member II has moved out of engagement with thecontacts I24, and I25 back into the position illustrated. This 180 degree rotation of the shaft I44 causes'the-doorsv 2 9 and 38 to move back to their original positions closing the openlugs" and 28 and reestablishing communica tionbetween the upper and lower portion of the fixture II; The cam I58 carried by the shaft I44 is also moved back to the position illustrated thus deenergizing the heater 22 and the fan motor 24 soas to stop the circulation of :heated air over the evaporator II. I I 1 The defrosting operation is now completedand the compressor I3 will operate untilthe temperature in the compartment I2 drops to the=desired value unless for some reason or other the=head pressure became sufliciently high as to open'the maintaining circuit at contacts 16 and 88 or unless the circuit to the compressor motor were opened at contacts 89 and I88 by reason of some overload condition. I The compressor will now cycle on and off as before to maintain the'te'mperature in the compartment I2 at the desired value until such time as the pressure within the system drops low enough to stop the compressor during one of its operations thus indicating-the existence of sufficient frost on the evaporator as to warrant the necessity of another defrosting operation. It will be recalled, however, that the compressor can at no timestart operating until the head pressure has dropped sufiiciently to permit the closure of contacts 19 and 8I,-thus insuring that the compressormotor will not have to start operation under a heavy load condition. It will thus be apparent thatI havedevised an arrangement wherein-automatic defrosting of the evaporator is initiated whenever necessary, in a system wherein a fixture temperature is to be maintained at a relatively low value, the-storage compartment of the fixture being isolated from the evaporator during each defrosting operation to prevent an undue temperature rise therein and at the same time a circulation of heated air over the evaporator is effected vto accelerate the defrosting operation so that the evaporator will be available for cooling the storage compartment I2 to the desired temperature within a relatively short time and before'the temperature therein has had an opportunityito increase to any appreciable degree. At the same time, assurance is had that theheadpressure in the refrigeration system will drop to a predetermined value between each operation of the compressor to safeguard the compressor motor againstahigh starting load. i I i Having described a preferred form'of my'invention, many modifications may become apparent to those skilled in the art. For example, in place of the heater22 and the=fan 23 the heater might be placed below the evaporator and the fan 28 dispenseduwith'i, It should be understood that the fan may be usedwithout the heater; or the heater without the fan. Furthermora'any other suitable arrangement for accelerating the defrosting of the evaporator might be'employed,

thereon, Also, any suitable motor meansmight be provided for operating the doors 29-and-l58. While the lrestarting of the" compressor after a defrosting'period is in'response tc the pressure in the evaporator,since thispressure is an indication of the evaporator temperature-At should be obvious5 that the response might be from a bulb mounted in engagement with the surface of the evaporator to indicate when the evaporator is defrosted. Many other modifications will also become apparent to those skilled in the art, and

I therefore wish it to be understood that my :invention is limited only by the scope of the appended claims. I

I claim as my invention:

1. In a refrigeration system having an evaporator, a condenser and a compressor for circulating refrigerant through the evaporator, a casing housing said evaporator, said casing including a storage compartment, means responsive to the temperature in said storage compartment for controlling operation of said compressor to maintain the temperature in said storage compartment at a desired value, means for interrupting operation of the compressor when the suction pressure therein drops to a predetermined value and preventing restarting of the compressor after it has been stopped by reason of this drop in suction pressure until the evaporator is defrosted, and means responsive to the stopping of the compressor by the drop in suction pressure for isolating the evaporator from the storage compartment to prevent undue rise in temperature in the storage compartment while the evaporator is being defrosted.

2. In a refrigeration system having an evaporator, a condenser and a compressor for circulating refrigerant through the evaporator, a casing housing said evaporator, said casing including a storage compartment, meansresponsive to the temperature in said storage compartment for controlling operation of said compressor to maintain the temperature in said storage com partment at a desired value, means for interrupting operation of the compressor when the suction pressure therein drops to a predetermined value and preventing restarting of the compressor after it has been stopped by reason of this drop in suction pressure until the evaporator is defrosted, means responsive to the stopping of the compressor by the drop in suction pressure for accelerating the defrosting of the evaporator, and automatic means for isolating portions of said compartment from said evaporator during defrosting.

3. In a refrigeration system, an evaporator for normally maintaining the temperature of a space below the freezing point of water, a compressor for circulating refrigerant through the evaporator, means responsive to the temperature of the space for normally controlling operation of the compressor, a second means in control of said compressor, means responsive to the stopping of said compressor by said second means for preventing restarting thereof until the evaporator has been defrosted, means responsive to the stopping of the compressor by said second means for accelerating the defrosting of said evaporator, and automatic. means for isolating portions of said space from said evaporator during defrost- 4. In a refrigeration system, an evaporator for normally maintaining the temperature of a space below the freezing point of water, a compressor for circulating refrigerant through the evaporator, means responsive to the temperature of the space for normally controlling operation of the compressor, a second means in control of' said compressor, means responsive to the stopping of said compressor by said second means for preventing restarting thereof until the evaporatorhas been defrosted, and means responsive to the stopping of the compressorby said second means for isolating the evaporator from the space being cooled.

5. In a refrigeration system having an evaporator, a condenser and a compressor forcirculating refrigerant through the evaporator, a casing housing said evaporator, said casing including a storage compartment, means responsive to the temperature in said storage compartment for controlling operation of said compressor to maintain the temperature in said storage compart ment at a desired value, means for interrupting operation of the compressor when the suction pressure therein drops to a predetermined value and preventing restarting of the compressor after it has been stopped by reason of this drop in suction pressure until the evaporator is defrosted, and means responsive to the stopping of the compressor by the drop in suction pressure for isolating the. evaporator from the storage oompartment and opening that portion of the casing housing the evaporator to the outside atmosphere.

6. In a refrigeration system having an evaporator, a condenser and a compressor for circulating refrigerant through the evaporator, a casing housing said evaporator, said casing including a storage compartment, means responsive to the temperature in said storage compartment for controlling operation of said compressor to maintain the temperature in said storage compartment at a desired value, means for interrupting operation of the compressor when the suction pressure therein drops to a predetermined value and preventing restarting of the compressor after it has been stopped by reason of this drop in suction pressure until the evaporator is defrosted, means responsive to the stopping of the compressor by the drop in suction pressure for isolating the evaporator from the storage compartment and opening that portion of the casing housing the evaporator to the outside atmosphere, and means for heating the evaporator during the defrosting period.

7. In a refrigeration system having an evaporator, a condenser and a compressor for circulating refrigerant through the evaporator, a casing housing said evaporator, said casin including a storage compartment, means responsive to the temperature in said storage compartment for controlling operation of said compressor to maintain the temperature in said storage compartment at a desired value, means for interrupting operation of the compressor when the suction pressure therein drops to a predetermined value and preventing restarting of the compressor after it has been stopped by reason of this drop in suction pressure until the evaporator is defrosted, and means responsive to the stopping of the compressor by the drop in suction pressure for isolating the evaporator from the storage compartment to prevent undue rise in temperature in the storage compartment while the evaporator is being defrosted, said last named means including a pair of doors movable between two positions, in one of which the interior of the casing 'is closed to the outside atmosphere and communication between the evaporator and the storage compartment is permitted, and in the other position of the doors, communication between the evaporator and the storage compartment is prevented and communi-. cation between that portion of the casing housing the evaporator, and the outside atmosphere is permitted.

8. In a refrigeration system having an evaporator, a condenser and a compressor for circulatin refrigerant through the evaporator, a casing housing said evaporator, said casing including a storage compartment, means responsive to the temperature in said storage compartment for controlling operation of said compressor to maintain the temperature in said storage compartment at a desired value, means for interrupting operation of the compressor when the suction pressure therein drops to a predetermined value and preventing restartin of the compressor after it has been stopped by reason of this drop in s uction pressure until the evaporator is defrosted, heating means located adjacent the evaporator, and means operative simultaneously with the stopping of the compressor by the dropjn suction pressure for energizing said heating means and for isolating the evaporator from the storage compartment to prevent undue rise in temperature in the storage compartment during the de frosting of the evaporator.

9. In a refrigeration system having an evaporator, a condenser and a compressor for circulating refrigerant through the evaporator, a casing housing said evaporator, said casing including a storage compartment, means for initiating operation of the compressor only when the suction pressure is above a low predetermined value and the head pressure is below a predetermined value, means responsive to the temperature in the storage compartment for continuing operation of the compressor as long as the temperature is above a predetermined value unless the suction pressure falls below said low predetermined'value or the head pressure .rises above a high predetermined value, means responsive to stopping of the compressor by reason of the suction pressure falling below said low predetermined value to prevent restarting of the compressor until the evaporator is defrosted, and means responsive to the stopping of the compressor by the drop in suction means responsive to the temperature in the storage compartment for continuing operation of the compressor as long as the temperature is above a predetermined value unless the suction pressure falls below said low predetermined value or the head pressure rises above a high predetermined value, a thermal switching mechanism including a pair of heating elements, means responsive to the heating of one of said heating elements for stopping operation of the compressor, means responsive to a drop in suction pressure below a predetermined value for energizing said one of said heating elements, means responsive to energization of the other of said heating elements for reenergizing said compressor, and means iesponsive to a rise in the suction pressure to a value which is a measure of the defrosting of the evaporator for energizing said other heating element.

12. In a refrigeration system having an evaporator, a condenser and a, compressor for circulating refrigerant through the evaporator, a casing housing said evaporator, said casing including a storage compartment, means for initiating operation of the compressor only when the suction pressure is above a low predetermined value and the head pressure is below a predetermined value, means responsive to the temperature in the storage compartment ,for continuing operation of the compressor as long as the temperature is above a predetermined value unless the suction pressure falls below said low predetermined value or the head pressure rises above a high predetermined value, a thermal switching'mechanism including a pair of heating elements, means responsive to the heating of one of said heating elements for stopping operation of the compressor, means responsive to a drop in suction pressure below a predetermined value for energizing said one of said heating elements means responsive to energization of the other of said heating elements for reenergizing said compressor, means responsive to a rise in the ing a'storage compartment, means for initiating 4 operation of the compressor only when the suction pressure is above a low predetermined value and the head pressure is below a predetermined value, means responsive to the temperature in the-storage compartment for continuing operation of the compressor as long as the temperature is above a predetermined value unless the suction pressure falls below said low predetermined value or the head pressure rises above a high predetermined value, means responsive to stopping of the compressor by reason of the suction pressure falling below said low predetermined value to prevent restarting of the compressor until the evaporator is defrosted, means responsive to the stopping of the compressor by the drop in suction pressure below the low predetermined value for isolating the evaporator from the storage-compartment, and means for circulating heated air over the evaporator while the evaporator is isolated from the storage compartment.

11. In a refrigeration system having an evaporator, a condenser and a compressor for circulating refrigerant through the evaporator, a casing housing said evaporator, said casing including a a storage compartment, means for initiating opsuction pressure to a value which is a measure of the defrosting of the evaporator for energizing said other heating element, and means responsive to the energization of the first of said heating elements for isolating the evaporator from the storage compartment-to prevent undue rise in temperature in the storage compartment while the evaporator is being defrosted.

13. In a refrigeration system, an evaporator, meansfor circulating a refrigerant through said evaporator, a casing housing said evaporator, said casing including a storage compartment, means responsive to the temperature ofsaid storage compartment for causing refrigerant to be circulated through said evaporator when the temperature in said compartment rises to a predetermined value and for interrupting the flow of refrigerant through said evaporator when the temperature in said compartment drops to a predetermined value, means for interrupting the flow of refrigerant through said evaporator when the pressure therein drops to a predetermined value and preventing flow of refrigerant therethrough until the pressure therein rises to a value which is a measure of the evaporator becoming defrosted, and means operative in response to the pressure in the evaporator dropping to said predemined value for isolating the evaporator from the storage compartment to prevent undue rise in temperature in the storage compartment whil r the evaporator is being defrosted.

14. In a-refrigeration system, an evaporator,

means for circulating a refrigerant through said evaporator, a casing housing said evaporator, said casing including a storage compartment, means responsive to the temperature of said storage compartment for causing refrigerant to be circulated through said evaporator when the temperature in said compartment rises to a predetermined value and for interrupting the flow of refrigerant through said evaporator when the temperature in said compartment drops to a predetermined value, means for interrupting the flow of refrigerant through said evaporator when the pressure therein drops to a predetermined value and preventing flow of refrigerant therethrough until the pressure therein rises to a value aaaaeoa providing communication.- between that portion of the casing housing the evaporator and the outside atmosphere; and means for simultaneing housing said evaporator, said casing including a storage compartment, means for causing operation of the compressor to maintain the temperature of the storage compartment at a predetermined value, means for interrupting operawhich is a measure of the evaporator becoming defrosted, and means operative in response to interruption in the flow of refrigerant by reason of the. pressure in the evaporator dropping to said predetermined value for isolating the evaporator from the storage compartment and for providing communication between that portion of the casing housing the evaporator and the outtion of the compressor whenever the suction pressure in the system drops to a sufliciently low value regardless of the temperature in the storage compartment and preventing restarting thereof until the suction pressure rises to a high predetermined value, and means for isolating the evaporator from the storage compartment in response to the stopping of the compressor'from suction pressure until the compressor is restarted.

17. In a refrigeration system having an evapj perature oftthe storage compartment at a prebe circulated through said evaporator when the a temperature in said compartment rises to a predetermined value and for interrupting the flow of refrigerant through said evaporator when the temperature in said compartment drops to a predetermined value, means for interrupting the flow of refrigerant through said evaporator when the pressure therein drops to a. predetermined value and preventing fiow of refrigerant therethrough until the pressure therein rises to a value which is a measure of the evaporator becoming defrosted, means operative in response to interruption in the flow of refrigerant by reason of the pressure in the evaporator dropping to said predetermined value for isolating the evaporator from the storage compartment and iior determined value, means for interrupting operation of the compressor whenever thesuction pressure in the system drops to a sufliciently low value regardless of the temperature in. the storage compartment and. preventing restarting thereof until the suction pressure rises to a' high predetermined value,.means for interrupting operation of the compressor whenever the head pressure rises to a predetermined value and preventing restarting of the compressor until the head pressure drops to a predetermined value, and means for isolating the evaporator from the storage compartment in response to the stopping of the compressor from suction pressure until the 

